Shatapatha Brahmana 3.7.1.27

3.7.1.27

yajñena vai devāḥ | imāṃ jitiṃ jigyuryaiṣāmiyaṃ jitiste hocuḥ kathaṃ na idam manuṣyairanabhyārohyaṃ syāditi te yajñasya rasaṃ dhītvā yathā madhu madhukṛto nirdhayeyurviduhya yajñaṃ yūpena yopayitvā tiro 'bhavannatha yadenenāyopayaṃstasmādyūpo nāma purastādvai prajñā purastānmanojavastasmātpūrvārdhe minoti

Verily, by means of the sacrifice the gods gained that supreme authority which they now wield. They spake, 'How may this (world) of ours be made unattainable to men?' They sipped the sap of the sacrifice, as bees would suck out honey, and having drained the sacrifice and scattered it by means of the sacrificial stake, they disappeared. And because they scattered (yopaya) therewith, therefore it is called yûpa (sacrificial stake). At the head stands intelligence, at the head swiftness of thought; therefore he sets it up on the fore-part (of the altar).